1. Field of the Invention
The present invention relates to an image recording apparatus and a calculation method which can obtain a distance from an optical sensor mounted on a carriage to a surface of a sheet on a platen, or a light reflectivity of the surface of the sheet, using the optical sensor constituted by only a pair of a light emitting element and a light receiving element.
2. Description of the Related Art
In an ink-jet recording apparatus, an image is recorded on a recording sheet by ejecting an ink from a recording head toward the recording sheet fed along a sheet-feed path. An optical sensor is mounted on a carriage with the recording head. As the optical sensor, there is generally used, e.g., a sensor including an LED as a light emitting element and a photo transistor as a light receiving element which are constructed integrally with each other. A light emitted from the light emitting element is reflected from the recording sheet or a platen which supports the recording sheet, and part of the light is received by the light receiving element and converted to a voltage. The platen is provided by a black member and has a lower light reflectivity than a surface of the recording sheet on which the image is not recorded. Thus, a position at which an output voltage outputted from the light receiving element is greatly changed is detected, whereby a position of an end portion of the recording sheet can be detected.
The above-described light emitted from the light emitting element reaches a reflecting surface while diffusing, and then part of the light reaches the light receiving element while further diffusing. An amount (a light receiving amount) of the light received by the light receiving element at this time is the largest in a case in which the light emitted from a center of the light emitting element along an optical axis reaches a center of the light receiving element. The light receiving amount decreases in accordance that a position at which the light reaches becomes distant from the center of the light receiving element. In other words, a ratio of the light received by the light receiving element in entire reflected light is determined by a distance between a barycenter of the reflected light (i.e., a central position of the reflected light in a plane of the light receiving element where it is assumed that all reflected light has reached the plane of the light receiving element) and the center of the light receiving element. Where the distance between the barycenter of the reflected light and the center of the light receiving element is constant, and light-emission intensity of the above-described light emitting element is constant, the output voltage outputted from the light receiving element is determined by a light reflectivity of the reflecting surface. The surface of the recording sheet is white or other bright colors and thus has a relatively high light reflectivity. In particular, a glossy sheet has a higher reflectivity than a normal sheet. Thus, the output voltage outputted from the light receiving element in a case in which the light emitted from the light emitting element is reflected from the glossy sheet is higher than the output voltage outputted from the light receiving element in a case in which the light emitted from the light emitting element is reflected from the normal sheet. Thus, a type of the recording sheet can be judged on the basis of a magnitude of the output voltage outputted from the light receiving element.
Further, where the above-described light-emission intensity of the light emitting element is constant, and the reflectivity of the reflecting surface is constant, the output voltage outputted from the light receiving element is determined by the distance between the barycenter of the reflected light and the center of the light receiving element.
Meanwhile, where a thickness of the recording sheet used in image recording is changed, a length of time in which the ink ejected from the recording head is attached to the recording sheet on the platen is changed. This cause a problem that an image recorded on the recording sheet is distorted because the ink is attached to a position displaced or distant from a desired position. To solve this problem, there is a need to perform proper image recording by a method that a timing at which the ink is ejected from the recording head is changed on the basis of the thickness of the recording sheet, for example. Thus, with reference to Patent Documents 1-4, a conventional image recording apparatus is provided with a means for detecting the thickness of the recording sheet supplied to the sheet-feed path.
A recording apparatus disclosed in Patent Document 1 (US 2007/0047157 A1 corresponding to JP-A-2007-91467) includes a multi-purpose sensor. This multi-purpose sensor is mounted on a carriage with a recording head and includes one infrared LED and two phototransistors. The infrared LED is a light emitting element disposed so as to emit light at 45° with respect to a measuring surface (i.e., a surface of a platen or a surface of a recording sheet). Each of the two phototransistors is a light receiving element disposed such that a light receiving axis is parallel to a central axis of a reflected light. Each of the two phototransistors is disposed in a state in which an optical axis of each phototransistor is displaced from the infrared LED. Thus, output voltages outputted from the two phototransistors are greatly changed in accordance with a distance from the multi-purpose sensor to the measuring surface. In this recording apparatus, where the output voltages of the respective phototransistors have been obtained, a distance coefficient is obtained on the basis of the two output voltages. The recording apparatus has a table representative of a relationship between the distance coefficient obtained by calculation and the distance from the multi-purpose sensor to the measuring surface. Thus, in the recording apparatus, the distance from the multi-purpose sensor to the measuring surface, i.e., a distance from the multi-purpose sensor to a surface of the recording sheet can be obtained on the basis of the table and the distance coefficient obtained in the above-described manner. A thickness of the recording sheet is obtained by subtracting the thus obtained distance from the distance from the multi-purpose sensor to the platen.
A recording apparatus disclosed in Patent Document 2 (JP-A-2007-62219) includes an optical sensor which is similar to the multi-purpose sensor disclosed in Patent Document 1. An uneven surface is formed in an end portion of a platen as a surface from which light emitted from an infrared LED is reflected. A sensitivity of two phototransistors is corrected on the basis of output voltages of the respective two phototransistors in a case in which projecting portions and depressed portions of the uneven surface are used as a reflecting surface. Thus, it is possible to obtain a thickness of a recording sheet more accurately than in the recording apparatus disclosed in Patent Document 1.
A recording apparatus disclosed in Patent Document 3 (JP-A-2006-168138) includes an LED as a light emitting element and a line sensor as a light receiving element. The LED and the line sensor are mounted on a carriage which is reciprocable in a direction perpendicular to a sheet-feed direction of a recording sheet. In this recording apparatus, light is emitted from the LED to the recording sheet fed along a sheet-feed path. Reflected light of this emitted light is received by the line sensor. A position of the line sensor at which an output voltage based on the reflected light is the largest is obtained on the basis of an output signal outputted from each element of the line sensor. Then, a distance between the recording sheet and the line sensor is obtained on the basis of this positional information. On the basis of this distance, a distance between the recording head and the recording sheet, and the thickness of the recording sheet are obtained.
A recording apparatus disclosed in Patent Document 4 (U.S. Pat. No. 7,441,849 B2 corresponding to JP-A-2006-305479) includes an LED as a light emitting element and a light receiving element. The light emitting element is disposed on an outside of an area in which a recording sheet is fed and emits light slantly relative to a platen. The light receiving element is mounted with a recording head on a carriage which is reciprocable in a direction perpendicular to a sheet-feed direction of a recording sheet. In this recording apparatus, the carriage is moved in a direction in which the light receiving element is moved toward or nearer to the light emitting element, and light is emitted from the light emitting element. Where the recording sheet supplied to the sheet-feed path has reached a position on the platen, the light emitted from the light emitting element is reflected not from the platen but from a surface of the recording sheet. A controller of the recording apparatus detects a position of the carriage when the light receiving element has received this reflected light. The position of the carriage when the light receiving element has received the reflected light emitted from the light emitting element is changed in accordance with a distance between the recording head and the recording sheet. Thus, the distance between the recording head and the recording sheet is obtained on the basis of the position of the carriage which has been detected in the above-described manner. A thickness of the recording sheet can be obtained on the basis of (a) this distance between the recording head and the recording sheet and (b) a distance from the recording head to the platen.